Home Economics 101

I'm fairly sure it's a landlord's legal responsibility to be sure a place is safe and habitable. Mold isn't safe, ergo, landlord has a responsibility to do something about it, imo. If your landlord doesn't seem to care, research what the landlord-tenant laws are in your area. Obviously, just because it's the law doesn't mean he'll do it, but it will give you a little more leverage to entice him to act.